This invention relates generally to the transmission and reception of data in spread spectrum systems and, more particularly, to improvements in digital processing techniques for use in a pseudo-noise (PN) receiver. Digital modulation techniques for communication are well known, and include phase shift keying (PSK), where a constant amplitude carrier signal is selectively reversed in phase to indicate a binary change of state of a data signal. In quadriphase phase shift keying (QPSK), the modulated carrier can assume any of four phase states, as determined by pairs of data bits.
For security and other reasons, a modulated carrier signal may also be subject to spread spectrum modulation. A spread spectrum signal is, as the name implies, spread over a wide bandwidth and is relatively immune to eavesdropping and jamming. A technique uses a pseudo-random (PN) code sequence to obtain the desired spectral spreading. A PN sequence is binary sequence that repeats itself after a large number cycles. Thus the binary numbers in the sequence are not truly random, but if the repetition cycle of the sequence is long enough its spectrum shares many of the properties of random electromagnetic noise. In the context of a data transmitter, PN modulation may be effected by simply passing the data stream and the PN code sequence through an exclusive OR gate, to achieve PSK modulation of the data onto the PN code. Data bits are either inverted or not, depending on the presence or absence of a logical "1" bit in the PN code. The data symbol rate is typically many times slower than the PN code rate (referred to as the PN "chip" rate). The resulting digital data stream is a PN code modulated by the slower data symbol stream, and is used to modulate a carrier signal in accordance with a digital modulation technique, such as QPSK, and the modulated carrier is transmitted. The present invention is concerned with systems of this general type, and particularly with such systems in which there may be multiple transmission paths between a transmitter and a receiver.
Receiving and demodulating signals that have been subject to PN modulation requires that the same PN code sequence be generated in the receiver, and correlated with received signals to extract the data modulation. One type of correlation technique employs a digital matched filter to compare the received digital signal with the locally generated version of the PN code. The digital filter produces an in-phase (I) signal and a quadrature (Q) signal from which a digital demodulator (such as a DPSK demodulator) can derive data values. Another function of the digital matched filter is to produce correlation measurements from which synchronization (sync) signals can be generated and used to handle multipath components in the received data signals. To better understand this aspect of PN-modulated data transmission, some further background is needed.
Multipath components arise in rf communication systems of various types when a receiving antenna detects signals arriving non-simultaneously over different paths. Multiple transmission paths may result from various causes, such as from atmospheric effects, or reflections from buildings or geographical features. In any event, a transmitted signal may produce multiple received signals of different strength. Conventionally, multipath errors are resolved in a PN correlator by selecting one or two correlation measurements having the highest signal strength, and using only these measurements during subsequent signal processing in which data demodulation is completed. For example, a PN correlator may generate an output spanning a few microseconds, long enough to produce multiple correlation output peaks resulting from multipath errors. Typically a single correlation peak value is detected in a sync detector, which integrates over a suitably large number of symbols, and a time epoch associated with the detected correlation peak is used to control input to a data demodulator.
A practical characteristic of transmissions involving multipath errors is that the multipath conditions may vary rapidly with time, especially if the transmitter or receiver, or both, are in motion, or if a source of multipath reflections is in motion. Therefore, a path that provides the maximum signal strength at a receiver at one instant in time may fade or disappear in the next instant, to be replaced by other signal paths providing different signal strengths. Ideally, it would be desirable to keep track of all the principal multipath signals, above some threshold, to provide continuity and quality of reception. Prior to the present invention, receivers attempting to achieve this goal have necessarily been extremely complex, bulky, and correspondingly costly to manufacture. No portable transceivers available prior to the present invention are believed to be capable of performing PN correlation measurements for multiple correlation peaks resulting from multiple transmission paths. The present invention is directed to this end.